A deal to head off a constitutional crisis over plans for a referendum on Scottish independence was emerging last night as the Conservatives, Labour and the Liberal Democrats joined forces to oppose the break-up of the United Kingdom.
The Government may allow Alex Salmond, the First Minister and leader of the Scottish National Party (SNP), to hold a referendum on independence at his preferred time of autumn 2014 in exchange for a straight Yes or No question on the ballot paper.
Mr Salmond would be forced to drop his planned third option, which would allow for the devolution of all powers, except foreign and defence policy, from London to Edinburgh. Known as "devo max", it is seen as an insurance policy if full independence is rejected by the Scottish people.
In London, ministers said the devo max question was a "red line" they would not cross. They will insist on a ballot in which Scots would choose between independence and remaining in the Union, even if this means a legal wrangle which may have to be resolved by the Supreme Court.
One Government source said last night: "You can't have a 'maybe' on the ballot paper. It could be a recipe for chaos."
Nick Clegg wants the anti-independence campaign to be led by a Labour figure such as Alistair Darling, the former Chancellor and MP for Edinburgh South-West. Mr Darling said yesterday he was ready to "play my part" in the No campaign. Sir Menzies Campbell, the former Liberal Democrat leader and MP for North-East Fife, is also likely to promote the No vote.
The Prime Minister and the Labour leader, Ed Miliband, were unified in the Commons yesterday over the future of the Union. Mr Cameron told MPs he passionately believed "we are stronger together than we would be by breaking apart", while Mr Miliband said pro-Union parties had to advance "the positive case about the shared benefits to us all of Scotland's part in the United Kingdom".
Yesterday, Mr Salmond stepped up his attack on the UK Government's intervention, telling Mr Cameron he should "butt out" of Scottish affairs.
The Prime Minister is due to meet the First Minister in the next few weeks to discuss the logistics of the referendum.